Method for operating a gas turbine and gas turbine system for carrying out the method

ABSTRACT

A method for operating a gas turbine uses a first fuel at full load. An improved emission behavior is achieved by operating the gas turbine ( 11 ) at partial load with a second fuel, which has a richer mix of higher-value hydrocarbons (C2+) with 2 and more carbon atoms per molecule like ethane (C 2 H 6 ) and propane (C 3 H 8 ).

This application claims priority under 35 U.S.C. § 119 to Germanapplication number 103 45 566.3, filed 29 Sep. 2003, the entirety ofwhich is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the technical field of gas turbines. Itconcerns a method for operating a gas turbine, as well as a gas turbinesystem for carrying out the method.

2. Brief Description of the Related Art

Usually burners such as the so-called EV burner (double cone burner,ref., for example, U.S. Pat. No. 5,081,844) and the so-called SEV burner(secondary EV burner, ref., for example, an article by F. Joos et al.Development of the sequential combustion system for the GT24/GT26 gasturbine family, ABB Review 4, p. 4-16 (1998) or U.S. Pat. No. 5,454,220)used in gas turbines are designed for full load operation. Nonethelessit might be necessary to operate the gas turbine at partial load. Thismight be because the network cannot absorb the generated energy or thegas turbine is started up or shut down (repairs, etc.). If no specialmeasures are taken, partial load operation results in an unfavorableemission behavior with regard to CO and NOx. Another problem withpartial load operation of a gas turbine concerns the residue of therelatively cool exhaust gas in relatively short combustion chambers.Cold gases and short dwell times result in low NOx emissions, butunfortunately also in increased CO/UHC emissions.

Publication U.S. Pat. No. 5,081,844 mentioned in the introductionproposes addressing the issue of emissions with partial loads byalternating the arrangement of small and large premix burners and havingthe small premix burners function as pilot burners. Publication U.S.Pat. No. 5,454,220 mentioned in the introduction proposes this procedurefor gas turbines with SEV burners.

The disadvantage with this kind of emissions optimization for partialload operation is the fact that the burners in the main combustionchamber must be specially configured in order to allow for thecorresponding operation. This procedure is difficult to realize inretrospect in existing gas turbines without any special burnerarrangement, and thus it is difficult to eliminate emissions issuesduring partial load operation.

SUMMARY OF THE INVENTION

One aspect of the present invention therefore is providing a method foroperating a gas turbine that results in a material improvement of theemission behavior during partial load operation without any structuralchanges to the gas turbine itself, as well as to provide a gas turbinesystem for carrying out the method.

A principle of the invention is that the gas turbine that uses a firstfuel at full load uses a second fuel at partial load to improve theemission behavior, with the second fuel having a richer mix ofhigher-value hydrocarbons (C2+) with 2 or more carbon atoms per moleculelike ethane (C₂H₆) and propane (C₃H₈) compared to the first fuel (thehigher-value carbons are hereinafter referred to as C2+).

In an exemplary manner gaseous fuels are used as a first and secondfuel.

As a rule it is possible to use two separate gas sources for providingthe first and the second fuel with the gas sources having different gascomposition with regard to the higher-value hydrocarbons (C2+), wherebythe two fuels are taken directly from the gas sources so that there is aswitch between the gas source with the lower share of higher-valuehydrocarbons (C2+) and the gas source with the higher share ofhigh-value gas carbons (C2+) when there is a switch from full to partialload. As an alternative it is possible to generate the two fuels basedon a different mixture of the gases in the two gas sources.

An alternative is to produce the first fuel from the second fuel byseparating higher-value hydrocarbons (C2+). Especially favorable is amethod in which the separated, higher-value hydrocarbons (C2+) areplaced in intermediate storage and are added to the second fuel duringpartial load operation. Due to space issues it might be beneficial toliquefy the separated, higher-value hydrocarbons (C2+) prior tointermediate storage, to intermediately store them as a liquid gas andto evaporate them prior to adding.

Another alternative is characterized in that the second fuel is producedfrom the first fuel by adding higher-value hydrocarbons (C2+). Theadded, higher-value hydrocarbons can be taken from a local reservoirthat is either a liquid gas storage tank, whereby the liquid gas takenfrom the liquid gas storage tank is evaporated in an evaporator prior toadding, or is a gas reservoir.

Furthermore it is possible to generate the first and second fuel basedon a third fuel whose content of higher-value hydrocarbons (C2+) rangesbetween the first and the second fuel. The first and second fuel isgenerated from the third fuel by separating and adding higher-valuehydrocarbons (C2+). The higher-value hydrocarbons (C2+) that areseparated from the third fuel when generating the first fuel are, in anexemplary embodiment, placed in intermediate storage, are taken fromstorage when generating the second fuel and are added to the third fuel.Due to spatial restrictions it might again be beneficial to liquefy theseparated, higher-value hydrocarbons (C2+) prior to placing them inintermediate storage, to store them as a liquid gas and to evaporatethem prior to adding.

Exemplarily, the share of higher-value hydrocarbons (C2+) for the secondfuel is approximately 10% to 30% higher than in the first fuel.

An exemplary embodiment of the gas turbine facility in accordance withthe principles of the present invention is characterized in that themeans for a controlled change of the composition of the fuel flowingthrough the main fuel feeding line comprise an auxiliary fuel feedingline that discharges into the main fuel feeding line, and that isconnected to a reservoir that contains higher-value hydrocarbons (C2+).A controllable valve is arranged in the auxiliary fuel feeding line forcontrolling the gas stream that flows through the auxiliary fuel feedingline.

The reservoir that contains the higher-value hydrocarbons (C2+) can be agas reservoir. It can also be a liquid gas reservoir, whereby anevaporator is arranged in the auxiliary fuel feeding line.

Another exemplary embodiment according to principles of the presentinvention includes a gas separating mechanism arranged in the main fuelfeeding line that separates higher-value hydrocarbons (C2+) from the gasthat flows through the main fuel feeding line and provides them to thereservoir that contains higher-value hydrocarbons (C2+). Much space issaved when the reservoir containing the higher-value hydrocarbons (C2+)is a liquid gas reservoir, and a gas liquefying mechanism is arrangedbetween the gas separating mechanism and the liquid gas reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail based on the exemplaryembodiments in connection with the drawing. The figures show thefollowing:

FIG. 1 shows a simplified scheme of a first exemplary embodiment of agas turbine system in accordance with the invention that can be used tocarry out the method in accordance with the invention;

FIG. 2 shows a simplified scheme of a second exemplary embodiment of agas turbine system in accordance with the invention with the respectiveload-based control that can be used to carry out the method inaccordance with the invention; and

FIG. 3 shows an enlarged section of the control blocks of the load-basedcontrol shown in FIG. 2.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

One principle of the present invention recognizes the fact that theemission behavior of a gas turbine can be positively influenced byadding C₂H₆ and higher molecular hydrocarbons (C2+). It is specificallyCO emissions that can be kept below the required emission values acrossa much larger partial load range than is currently possible.Furthermore, this also has a positive influence on the extinguishinglimit, which in turn results in lower NOx emissions.

A suitable gas turbine system is shown in a simplified scheme in FIG. 1.The gas turbine system 10 comprises a gas turbine 11 that is comprised,as is customary, of a compressor 12, a combustion chamber and a turbine14. Compressor 12 and turbine 14 are arranged on a mutual shaft 15 thatis used to drive a generator, not shown in FIG. 1, for power generationpurposes. During operation the compressor 12 intakes the air, compressesit and feeds it into the combustion chamber 13, where it is mixed witha, for example, gaseous fuel, and the fuel is combusted. The hotcombustion gases from the combustion chamber are expanded in the turbinewith thermodynamic performance and then are reused (e.g., in HeatRecovery Steam Generator, HRSG) or are released to the outside. The fuelrequired in the combustion chamber 13 is taken from a fuel line 16(e.g., a natural gas pipeline) and is supplied to the combustion chamber13 via a main fuel feeding line 17. The supply can be controlled using afirst controllable valve 18.

An auxiliary fuel feeding line 19 discharges into the main fuel feedingline 17. It is connected to a liquid gas storage tank 22 via a secondcontrollable valve 20 and an evaporator 21. The liquid gas storage tank22 contains liquid gas 23 in the form of liquefied, higher-valuehydrocarbons (C2+). The liquid gas storage tank 22 can either be filledfrom the outside or—as is indicated by the dash-doted line in FIG. 1—canbe connected to a gas separating mechanism 27 via a gas liquefyingmechanism 28. The gas separating mechanism 27 is arranged in the mainfuel feeding line 17, and its design is such that it can withdraw a setamount of the higher-value hydrocarbons (C2+) from the gaseous fuel thatflows through the main fuel feeding line 17.

As an alternative to the liquid gas storage tank 22 it is possible touse a (e.g., underground) gas reservoir 24 as a reservoir forhigher-value hydrocarbons. It can be filled via a filling port 26 and aliquid gas tanker, for example, with subsequent evaporation.

The operation of the gas turbine system 10 can be described as follows:The gas turbine 11 is started with the customarily available, usuallygaseous fuel from the fuel feeding line and is connected to the networkusing the so-called pilot operation. As soon as the gas turbine 11 iscoupled to the network and provides increasing power to the network,higher-value hydrocarbons (ethane C₂H₆, propane C₃H₈, etc.) are added tothe fuel. This additive of so-called C2+ gas (with 2 and more C-atoms)is guided from the liquid gas storage tank 22 via the evaporator 21 andis added to the main fuel. A C2+ gas share of approximately 10 to 30% isadded in a controlled manner. As an alternative it is possible to addthe C2+ gas from the gas reservoir 24 to the fuel in a controlledmanner.

If a flame temperature is reached that is accordingly high, operation isswitched to the so-called premix operation. The so-called switchingtemperature at which operation can be switched to the premix operationstrongly depends on the (C2+) gas content at which the gas turbine 11 isoperated. The higher the (C2+) gas content, the lower the switchingtemperature can be set. The advantage of this operating procedure isthat the load range with premix operation and thus with low NOx emissioncan be increased significantly by adding (C2+) gas. The result is thatthe operation of the gas turbine 11 significantly reduces the entire NOxemissions.

As already mentioned in the introduction, another issue with partialload operation of a gas turbine is the burn out of the relatively coolexhaust gases in relatively short combustion chambers. Cold gases andshort dwell times result in low NOx emissions, but unfortunately also inincreased CO/UHC emissions (UHC=Unburned Hydro Carbons).

Here, too, it is found that adding (C2+) gas to the fuel results in afaster burn out of the exhaust gases. Despite relatively shortcombustion chambers with correspondingly short dwell times, the toxiccarbon monoxide is reduced to nontoxic CO₂ over a considerably largerpartial load range.

With increasing load the combustion temperatures approach full loadoperating temperatures. The combustion chamber 13 that is designed forfull load operation now has sufficiently high temperatures, and theburnout from CO to CO₂ occurs in a period of time that is shorter thanthe dwell time of the exhaust gases in the combustion chamber 13. The(C2+) gas content in the fuel can be accordingly reduced with increasingload until no more (C2+) gas from the liquid gas storage tank 22 or thegas reservoir 24 must be added.

The filling of a liquid gas storage tank 22 is the easiest. Customaryinfrastructure for filling liquid gas storage tanks is known and doesnot require any additional explanation at this point (supply of liquidgas with tanker/truck etc.). A gas reservoir 24 can be supplied withliquid gas as well. Before charging the gas reservoir 24 via the fillingport 26, the liquid gas is returned to the gaseous state using anevaporator.

However, a different method is also feasible for a gas turbine system 10according to FIG. 1: Natural gas as main fuel for gas turbines may havedifferent compositions depending on the location where the natural gasis produced. Each gas displays a different combustion behavior that isbased on its origin and composition. Not all “gas types” combustequally. However, there are regulations that require that emissions bewithin tight limits across the largest possible operating ranges.

This requirement cannot be met if different gases are combusted in thesame manner. On the other hand it is possible to optimally design andoperate the combustion system for a specifically defined gas. However,the gas to be combusted must correspond to the respective definitionused for the design. This can be achieved by separating the fuel gas. Tothis end, a gas separating mechanism 27 (dashed line in FIG. 1) isintegrated in the main fuel feeding line 17 to the gas turbine 11. Itensures that a fuel gas that meets the specification is added to the gasturbine 11. Any excess of (C2+) gas is withdrawn from the gas when thegas turbine 11 is operated at full load. The excess is liquefied in agas liquefying mechanism 28 and is placed in intermediate storage in theliquid gas storage tank 22 via a filling line 29 until the gas turbine111 is/must be operated in the partial load operation, and a lack of(C2+) must be offset instead of a surplus of (C2+). The withdrawn gascan also be sent to a suitable underground gas reservoir 24 via afeeding line 25. The gas reservoir 24 or liquid gas storage tank 22 isunloaded during the corresponding partial load operation. The size ofthe intermediate reservoir 22, 24 must be chosen based on theanticipated partial load and full load hours and the gas composition,

If different gas sources with different gas compositions are used foroperating the gas turbine, it is furthermore possible to use smallerintermediate storage (liquid gas storage tank or gas reservoir). Duringpartial load operation the gas turbine is operated with the (C2+) richerfuel gas with empty intermediate storage, and at full load (orcorrespondingly high partial load) operation is switched to the (C2+)poorer gas source, or the gas turbine is operated with the C2+ rich gasduring full load and excess (C2+) gas is separated via a gas separatingmechanism again and stored in intermediate storage. When operating withtwo gas sources, it might be advantageous from time to time to mix thetwo gases at a certain ratio and to add the mixture to the gas turbineaccording to the (C2+) content that is required at the time and in orderto be able to optimally operate the respective operating point (low COand low NOx emissions).

An example of a gas turbine system that works with two gas sources isshown in FIG. 2. The gas turbine system 30 of FIG. 2 comprises a gasturbine 31 with a compressor 32, two combustion chambers 35 and 36, afirst turbine 33 arranged between them and a second turbine 34. Theturbines 33, 34 and the compressor 32 are arranged on a mutual shaft 37that drives a generator 38. The configuration corresponds to the oneshown in U.S. Pat. No. 5,454,220. The combustion chambers 35, 36 aresupplied with gaseous fuels via a main fuel feeding line 51, 52 from twogas sources in the form of two gas pipelines GPI and GPII. It ispossible to produce a predefined mixture of both fuels in a precedingmixing device 46 with two control valves 47, 48, or it is possible toswitch to a different fuel. (C2+) shares can be separated in asubsequent gas separating mechanism 45 and can be placed in a reservoir43 via a compressor 42. The separated fuel is compressed in a compressor41 and guided to the combustion chambers 35, 36 via the two main fuelfeeding lines 51, 52. On each of the two main fuel feeding lines 51, 52it is possible to add higher-value hydrocarbons (C2+) from the reservoir43 via an auxiliary fuel feeding line 53, 54 and a system of nonreturnvalves 40 a, . . . ,d and control valves CV3, CV4. The fuel mass flowrate in the respective main fuel feeding line 51, 52 can be controlledwith a control valve CV1 and/or CV2. The reservoir 43 can also be filledexternally from a filling station 44.

The various fuel mass flow rates indicated in FIG. 2 and measured at therespective locations ({dot over (m)}_(x), x=I,II,1,2,CV1, . . .,CV4;C2+ext,Res) are controlled with a load controller 39 via twocontrol devices 49, 50 based on the power or load required from thenetwork, with the devices on one hand controlling the entire fuel massflow rate per combustion chamber 35, 36 via control valves CV1 and CV2,and on the other hand controlling the adding of (C2+) shares to the fuelvia control valves CV3 and CV4. The respective exemplary control curvesare indicated in the enlarged presentation of the control devices 49, 50in FIG. 3. The following applies:{dot over (m)} _(CV1) =f(load){dot over (m)} _(CV2) =f(load){dot over (m)} _(I) +{dot over (m)} _(II) −{dot over (m)} _(C2+ext)={dot over (m)} _(CV1) +{dot over (m)} _(CV2){dot over (m)} _(Res) ={dot over (m)} _(C2+ext) −{dot over (m)} _(CV3)−{dot over (m)} _(CV4)m_(Res) =∫{dot over (m)} _(res) ·dt.

List of Reference Numerals

10, 30 gas turbine system 11, 31 gas turbine 12, 32 compressor 13, 35,36 combustion chamber 14, 33, 34 turbine 15, 37 shaft 16 fuel line 17,51, 52 main fuel feeding line 18, 20 valve 19, 53, 54 auxiliary fuelfeeding line 21 evaporator 22 liquid gas storage tank 23 liquid gas 24gas reservoir 25 feeding line 26 filling port 27, 45 gas separatingmechanism 28 gas liquefying mechanism 29 filling line 38 generator 39load controller 40a, . . . , d nonreturn valve 41, 42 compressor 43reservoir 44 filling station 46 mixing device 47, 48 control valve 49,50 control device CV1, . . . , CV4 control valve GPI, GPII gas pipeline

While the invention has been described in detail with reference toexemplary embodiments thereof, it will be apparent to one skilled in theart that various changes can be made, and equivalents employed, withoutdeparting from the scope of the invention. Each of the aforementioneddocuments is incorporated by reference herein in its entirety.

1. A method for operating a gas turbine using a first fuel at full load,the method comprising: operating the gas turbine during partial loadoperation with a second fuel instead of the first fuel to achieve animproved emission behavior, with the second fuel having a richer mix ofhigher-value hydrocarbons with 2 or more carbon atoms per molecule thansaid first fuel.
 2. A method according to claim 1, wherein the share ofhigher-value hydrocarbons is approximately 10% to 30% higher in thesecond fuel than in the first fuel.
 3. A method according to claim 1,wherein the second fuel comprises a fuel selected from the groupconsisting of ethane (C₂H₆) and propane (C₃H₈).
 4. A method foroperating a gas turbine using a first fuel at full load, the methodcomprising: operating the gas turbine with a second fuel instead of saidfirst fuel to achieve an improved emission behavior, with the secondfuel having a richer mix of higher-value hydrocarbons with 2 or morecarbon atoms per molecule than said first fuel; wherein the first fueland second fuel each comprise gaseous fuel.
 5. A method according toclaim 4, comprising: supplying the first fuel and the second fuel fromtwo separate gas sources, the first fuel and the second fuel havingdifferent gas compositions with regard to said higher-value hydrocarbon.6. A method according to claim 5, comprising: supplying the two fuelsdirectly from said gas sources; and when switching from full load topartial load, switching between a gas source with a low share ofhigher-value hydrocarbon and a gas source with a higher share ofhigh-value hydrocarbon.
 7. A method according to claim 5, comprising:generating the two fuels based on a different mixture of the gases fromthe two gas sources.
 8. A method according to claim 4, comprising:producing the first fuel from the second fuel, including separating thehigher-value hydrocarbon.
 9. A method according to claim 8, comprising:placing the separated higher-value hydrocarbons in intermediate storage;and adding said separated higher-value hydrocarbons to the second fuelduring partial load operation.
 10. A method according to claim 9,comprising: liquefying the separated higher-value hydrocarbons prior tointermediate storage; placing said liquefied separated higher-valuehydrocarbons in intermediate storage as liquid gas; and evaporating saidliquefied separated higher-value hydrocarbons prior to said adding. 11.A method according to claim 4, comprising: generating the second fuelfrom the first fuel, including adding higher-value hydrocarbons.
 12. Amethod according to claim 11, comprising: taking the added higher-valuehydrocarbons from a local reservoir.
 13. A method according to claim 12,wherein the local reservoir comprises a liquid gas storage tank; andcomprising: evaporating liquid gas from the liquid gas storage tank byan evaporator prior to said adding.
 14. A method according to claim 12,wherein the local reservoir comprises a gas reservoir.
 15. A methodaccording to claim 4, comprising: generating the first fuel and secondfuel from a third fuel whose content of higher-value hydrocarbons rangesbetween those of the first fuel and the second fuel, including eitherseparating the first fuel and the second fuel from the third fuel oradding higher-value hydrocarbons to the third fuel.
 16. A methodaccording to claim 15, comprising: placing the higher-value hydrocarbonsthat are separated from the third fuel when generating the first fuel inintermediate storage; removing higher-value hydrocarbons from saidintermediate storage; and generating the second fuel, including addingsaid removed higher-value hydrocarbons to the third fuel.
 17. A methodaccording to claim 16, comprising: liquefying the separated higher-valuehydrocarbons prior to said placing into intermediate storage; whereinsaid placing comprises placing said liquefied separated higher-valuehydrocarbons in intermediate storage as a liquid gas; and evaporatingliquefied separated higher-value hydrocarbons from intermediate storageprior to said adding.
 18. A method according to claim 4, wherein theshare of higher-value hydrocarbons is approximately 10% to 30% higher inthe second fuel than in the first fuel.
 19. A method according to claim4, wherein the second fuel comprises a fuel selected from the groupconsisting of ethane (C₂H₆) and propane (C₃H₈).